Like most area’s of this exercise stuff there is a whole lot of contradictory and if this and if that type of messages that come with your question.
I think really the best general answer is stretching at anytime is good.
Looking specifically day versus night there is certainly a strong case that could be made for both, which are built around the same principle : posture.
First for more morning, lets take a look at “why” we should. Number one. is that we just finished. While that may be a state of “relaxation,” in terms of your brain shutting down for a bit that doesn’t necessarily mean that our muscular system isn’t working. Sure, we are moving, but people don’t necessarily sleep in ideal positions. I mean, think about what “stiff neck,” is. The tendency for people is to sleep in positions that will reinforce dysfunctional postures. How many people have you had over the years with shoulder impingement type complaints that still sleep on that side of the body? Or better yet, how many people will sleep without elevating their knees and allow their lumber to stay in lordosis for eight hours a night? So for me it doesn’t make sense to just allow some to (1) not know how to sleep correctly but also (2) go from a position of stiffness to a day full of likely stiffness. Stretch in the morning. This doesn’t mean while you are still in bed. Get up brush your teeth, read your paper, shower, then stretch. Its that easy.
So we get out of bed and then we are off doing what we do. Most people don’t move in multi-planes equally and consistently enough. In fact, we live in a world that is dominated by the sagittal plane and SITTING. Obviously, that will create some muscle length tension relationships that will be routinely addressed. So why not use techniques during the day and at night. I mean, if we are respsonsible with our automobiles enough to take it to have the alignment adjusted, why would just let out bodies get stuck in positions that they shouldn’t be?
The answer is daily maintenance. Brush your teeth, comb your (if you have any!!) and stretch your limitations!
hope this helps,